I have never found Christie mysteries difficult to figure out, purely using the "narrative" method.

I find this difficult to believe, but, whatever. I understand what you're getting at.

Thanks to you, actually, I've decided to give the Pony Theory some thought, and... it works. It actually works.

Ryukishi spent his last interview talking about a "key" that would "unlock" all the closed rooms, and... what if the Pony Theory is it? That is, that Beatrice is in fact a physical existence on the island separate from anyone else? What if BOTH Shkanon and Erika-Doesn't-Exist are true? That leaves a slot open for another person, doesn't it? Sure, it's the Mysterious Person X all over again, but now it's the Mysterious Person Beatrice, which at least has some precedent.

Furthermore, it would explain something that's been bothering me for quite some time now. Assume Shkanon is true. Let's also assume that all the victims of Ep1's first twilight are, in fact, dead. How else do you explain the person Battler heard exit the boiler room during Ep1's fourth and fifth twilights? Mysterious Person Beatrice ties that up nicely.

With the Pony Theory, also, we can take the challenge letters completely at FACE VALUE. We can take everything attributed to Beatrice at face value. The theory suddenly clears up things that we always assumed were mysteries.